Formalize the notion of messing with sampler state.

Formalize the notion of messing with sampler state.

This kills a lot of the assumptions that have been going around,
and should allow us to deal much better with the situation when
we have two or more inputs to an effect (where you basically can't
predict the sampler number used reliably); there's still an edge
case that's documented with a TODO, but this is generally much better.